Digital cameras are ubiquitous. They are used in a wide range of devices, from expensive and complex equipment to cell-phones and webcams. A digital still or video camera includes at least an imaging lens and an image sensor. Typically the image sensor is a complementary-metal-oxide-semiconductor (“CMOS”) image sensor, but a charged coupled device (“CCD”) image sensor is also possible. The imaging lens forms an image at the image sensor. The image sensor typically includes millions of pixels or light sensing elements sensitive to one of three primary colors, e.g., red, green, and blue. Each pixel detects and transforms the light intensity at the pixel into an electric signal. Accordingly, a color image formed by the imaging lens is detected and transformed into electric signals. In other words, an optical color image is transformed by the image sensor into an electronic color image.
In contrast to the human eye, CMOS and CCD image sensors are typically also sensitive to near infrared light. Near infrared light, or for simplicity, IR light as referred in this disclosure, will also be detected. However, CMOS and CCD image sensors are typically not sensitive to IR light beyond the near infrared spectrum. Detected IR image generates errors in a displayed image, which is produced in the three primary colors, e.g., red, green, and blue.
To eliminate or reduce errors in the produced primary color images caused by the detected IR image, an IR-cut filter (“IRCF”), also known as an IR cut-off filter, is disposed between the imaging lens and the image sensor, such that IR light is blocked by the IR-cut filter while visible light is transmitted through the IR-cut filter. Thus, the use of an IR-cut filter achieves more realistic colors in white light. However, IR-cut filters can cause other problems as described in the following sections.